A typical thermoshaping press has upper and lower platens carrying respective tool or die halves. One of the die halves can be moved toward the other or both of them can be moved together to grip, heat, and deform a sheet workpiece, typically a thermoplastic synthetic-resin foil. In a standard system the sheet is formed with an array of upwardly open cups or pockets that are subsequently filled with a product, e.g. pills or yoghurt, and then a cover sheet, which can be plastic or a metal foil, is laminated over the upwardly open pockets or cups, and finally the laminate is cut into individual packages.
The press can serve to form the pockets, and can also be used to either actually punch out the pockets and a flange from the workpiece, or to score the workpiece so deeply between the pockets as to allow them to be readily separated from one another. Such a system is invariably part of a production line where cycling speed is very critical.
Swiss patent 399,724 describes such a system where the die halves, which can be used for deforming or punching, are shifted by respective pneumatic cylinders. To this end each platen is slidable along several guide rods. A knee linkage comprised of two: rigid links serves to vertically reciprocate the platen. Each link has an outer end pivoted either on the platen or on the machine frame and an inner end pivoted on the inner end of the other link. The piston rod of the pneumatic actuator is pivoted on the inner ends of the links so that it can flex the linkage and in effect push the outer ends apart or pull then together to shift the platen. Such a system is fairly slow.
It is known from DE 36 04 255 A1 to use a spindle system to implement the drive mechanism for opening and closing the tool, at least one ball screw being used. The spindle is driven by a drive motor. One end of the spindle is threaded into a big nut pivoted on the inner ends of the links, thereby permitting operation of the toggle link mechanism, and thus opening and closing of the tool, by axial displacement of the spindle. The tool part, which is linked to the toggle link mechanism, may thus be moved into its required positions. The spindles are usually designed as ball screws or planetary roller screws, which has the disadvantage that the maximum rotational speed of the spindle during operation is limited. The tool can be opened and closed only with a certain maximum threshold speed, which in turn limits the cycle time of the thermoshaping machine and thus the output of finished articles.
Another disadvantage is that the roller screw drives must be located in the middle of the tool or tool platform on account of the necessary central introduction of force. However, for some models of thermoshaping machines the prestretcher, which performs a mechanical preshaping of the heated plastic foil, for example by means of an air blast pushing the softened foil into die cavities, must be situated at this location. The prestretcher is required in particular for tall articles, and to reduce the complexity the prestretcher is not integrated into the tool, but instead must be positioned in the middle, i.e. centrally, in the thermoshaping machine, resulting in the need for appropriate space at this location. As a rule, the prestretcher must be able to perform a motion that is independent of the motion of the tool platform.
Another design of the drive mechanism for a tool part provides that, by means of a gear unit having a downstream crank mechanism, a servomotor actuates a knee-lever linkage which likewise moves the tool platforms into the required position. Both of the identically directed knee-lever linkages are simultaneously moved in the same direction by means of a connecting rod.
This knee-lever linkage has the disadvantage that the two identically directed toggle link pairs must move perpendicular to the opening and closing motion of the tool part by means of a single drive. This produces inertial forces which act perpendicular to the opening and closing motion of the machine, resulting in tilting of the entire molding or stamping station. The guide columns for the tool parts are not able to acceptably absorb these forces and deformations. The adverse effects are not only greater wear on the guide columns and bushings, but also significantly reduced service life of the steel strips used for cutting out the articles. Maintaining the two tool halves in a parallel configuration, which is extremely important when working with steel strips, is impaired by the high lateral inertial forces. Thermoforming machines of this type can therefore be operated only with a limited cycle frequency. A further disadvantage of this design is that the machine must be modified for different opening and closing strokes.